Article http://pubs.acs.org/journal/aesccq
Cation Exchange in the Presence of Oil in Porous Media R. Farajzadeh,*,†,‡ H. Guo,‡ J. van Winden,† and J. Bruining‡ †
Shell Global Solutions International, 2288 GS Rijswijk, The Netherlands Delft University of Technology, 2628 CD Delft, The Netherlands
‡
ABSTRACT: Cation exchange is an interfacial process during which cations on a clay surface are replaced by other cations. This study investigates the effect of oil type and composition on cation exchange on rock surfaces, relevant for a variety of oil-recovery processes. We perform experiments in which brine with a different composition than that of the in situ brine is injected into cores with and without remaining oil saturation. The cation-exchange capacity (CEC) of the rocks was calculated using PHREEQC software (coupled to a multipurpose transport simulator) with the ionic composition of the effluent histories as input parameters. We observe that in the presence of crude oil, ion exchange is a kinetically controlled process and its rate depends on residence time of the oil in the pore, the temperature, and kinetic rate of adsorption of the polar groups on the rock surface. The cation-exchange process occurs in two stages during two phase flow in porous media. Initially, the charged sites of the internal surface of the clays establish a new equilibrium by exchanging cations with the aqueous phase. At later stages, the components of the aqueous and oleic phases compete for the charged sites on the external surface or edges of the clays. When there is sufficient time for crude oil to interact with the rock (i.e., when the core is aged with crude oil), a fraction of the charged sites are neutralized by the charged components stemming from crude oil. Moreover, the positively charged calcite and dolomite surfaces (at the prevailing pH environment of our experiments) are covered with the negatively charged components of the crude oil and therefore less mineral dissolution takes place when oil is present in porous media. KEYWORDS: Cation exchange, Wettability, Rock-fluid interaction, Wetting film, Heterogeneous charge, Clays, Geochemistry
1. INTRODUCTION The efficiency of improved or enhanced oil recovery (IOR/ EOR) processes is often influenced by the composition of the flowing aqueous phase and is thus affected by the mass exchange between fluid and solid phases. For example, the rheology of polymers or the magnitude of the interfacialtension reduction by surfactants strongly depends on the ionic strength and hardness (concentration of divalent cations) of the aqueous phase.1,2 In recent years, the additional oil extracted by tuning the composition of the injected water has revived the interest in a more detailed understanding of the nature of the interactions between the rock and the fluids residing in the pore.3−18 Several interfacial phenomena occur simultaneously during two-phase flow in porous media, the extent of which depends on the compositions of the flowing (aqueous and oleic) and the stationary (rock) phases and the contact area affected by the surface roughness or irregularities. Indeed, reservoir rocks consist of irregularly shaped pores and grain assemblages with sharp edges and corners.19 The asperities and sharp edges are the “pinning points” at which crude oil contacts the rock20−23 possibly with a very thin water film in between held together by molecular forces on the surface.23−30 On other surfaces, such as quartz, the oil is separated from the rock by a “thicker” nonuniform water film.19 The stability of the water films depends on the equilibrium between interaction of double layer forces and van der Waals forces.25,29 A double layer occurs when a charged surface is in contact with an aqueous phase. The ions © XXXX American Chemical Society
with an opposite charge will have a higher concentration than the bulk concentration near the surface, whereas ions with a charge of the same sign will have a concentration less than the bulk concentration near the surface. When the oil−water surface and the rock−water surface have the same sign, it can be shown that the double layers repel each other, which is conducive to a stable water layer. The presence of high salt concentrations shields the surface charges leading to a reduced double-layer repulsion.29−31 The Debye length, κ−1 [m] is the characteristic distance over which a charge is shielded by the ions in a solution and is given by31 κ −1 =
ϵrϵ0kBT 2NAe 2I
= 9.63 × 10−9
1 I
(1)
where I = Σ 1/2 is the ionic strength [mol/m ], εr (20 °C) = 80.1 [-] is the relative permittivity, ε0 = 8.854 × 10−12 [F/m = C2/J/m ] is the permittivity of free space, kB = 1.38 × 10−23 [J/K] is the Boltzmann constant, NA = 6.0225 × 1023 is Avogadro’s number, and e = 1.602 × 10−23 C is the charge of an electron. In the case of the presence of a water film, van der Waals forces are generally attractive because the dielectric coefficient of water exceeds that for rock and oil and cizi2
Received: Revised: Accepted: Published: A
December March 14, March 16, March 16,
3
21, 2016 2017 2017 2017 DOI: 10.1021/acsearthspacechem.6b00015 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
Article
ACS Earth and Space Chemistry
with the formation brine. When the composition of the brine is altered, the cation exchanger readjusts its composition to a new equilibrium with the injected brine. The exchanger acts as a temporary buffer and may alter the brine composition through a process known as ion chromatography.40−42 If the resulting salinity is too high or the capillary pressure is high,29 the water film covering the rock surface becomes unstable resulting in a direct contact of oil with the rock surface (albeit often in presence on a thin hydration layer). On the other hand, if the salinity is low dominance of repulsive forces results in expansion of the double layer and eventually stabilizes the water films on the rock surface. If salinity becomes too low, simultaneous effects of (viscous) drag and electrostatic forces mobilize some fines and/or deflocculate clays (interlayer forces between clay layers that control the swelling state and integrity are mainly electrostatic interactions), which might be entrapped and block the rock pores.14−17 The charge density and distribution on the alumina-like surfaces of the clay also varies because of the cation-exchange reactions. The effect of cation exchange on oil recovery was first applied to surfactant flooding43−45 and has been since considered as an important component of the chemical enhanced-oil-recovery processes.46−50 Lager et al.7 argued that the cation exchange between the mineral surface and the invading brine can desorb oil from the rock surface. Despite its relevance, the effect of oil on cation exchange has not been carefully studied. Most of the knowledge on this topic is inferred from indirect measurements such as the amount of produced oil (in the case of low-salinity-water flooding) or delays in breakthrough of the injected alkali (in the case of alkali-surfactant−polymer flooding).47,51 All the experiments on this topic have been exclusively conducted without crude oil. However, as we will show in this study, the presence of oil and the dynamics of the interactions between the crude oil, brine, and the rock can lead to different results than those inferred from the single-phase experiments. This can in return pose questions to the direct translation of the coreflood experiments to field-scale water- or chemical-flooding projects. We hypothesize that (1) adsorption of basic components of the crude oil on the cation exchange sites reduces the cation exchange capacity of the rock, and (2) adsorption of acidic components of the crude oil on the calcite and dolomite inhibits the dissolution of these minerals at elevated temperatures. It is therefore our objective to investigate the effect of the oil type and composition on the extent and nature rock-fluid interactions in porous media. We perform experiments in which brine with a different composition than that of brine initially in the cores is injected into these cores with and without the remaining oil saturation. The CEC of the rocks was calculated using the ionic composition of the effluent histories. Single-phase experiments were conducted to obtain the cationexchange capacity of the cores, denoted by Qv (mequiv/mL pore volume), in the absence of the oil. The two-phase experiments were conducted with and without aging the cores with crude oil. When the cores were not aged at room temperature, the effect of oil on the CEC was insignificant; however, at a higher temperature the polar components of the oil started to interact with the oil. A kinetic behavior was observed for this process. This was confirmed by the reduced CEC of the core that was aged with the crude oil. In this case, because the oil was in contact with the rock for a longer period, the positively charged components of the oil exchanged with the cations adsorbed on the rock. The structure of the paper is
because the refractive index of water is generally less than for rock and oil (see Figure 1). These two forces suffice if the
Figure 1. Schematic of an oil drop inside irregular pores. The red lines represent the “pinning points”20 or “welding spots”,24 that is, charged mineral edges and asperities where oil can directly contact rock (in the presence of a hydration layer). The yellow lines represent the quartz surface and the silica-like surface of clays on which a thick nonuniform water film separates oil from the rock surface.
thickness of the water film considerably exceeds the molecular size or surface roughness; otherwise structural forces have to be considered. The force per unit surface area is called the disjoining pressure. If the capillary pressure is less than the local maximum disjoining pressure at large film thickness, the water film is stable. The surface charge can be calculated by considering the relevant chemical surface complexation reactions23,32−34 and can be found using PHREEQC.35 The charge of the oil−water surface is determined by the presence (or more strictly ionization) of functional groups such as carboxylic acids (negative charge) and basic compounds such as pyridines and quinolones (positive charge) at a given pH.28 A larger part of the rock surface is covered with a water film of variable thickness, depending on the ionic strength and pH of the flowing solution; however, some “asperities” will stick out of the water layer into the crude oil. The diffusion of polar components of the crude oil into the water film may lead to direct contact of oil with the rock and alteration of the wetting state of the surface similar to “spot-welding”.20−22 Depending on the nature of the charges on the rock surface (mineral type) and components of the crude oil and water, oil can be attached to the rock surface through several mechanisms such as ligand exchange, cation bridging, anion and cation exchanges, hydrogen bonding, van der Waals interaction, and so forth.7 These mechanisms require polar groups in the crude oil and their relative contribution depends on the composition of the phases; in particular, the composition of the aqueous phase is strongly affected by the geochemical reactions. Cation exchange is an interfacial process during which a cation on, for instance, a clay surface is replaced by another cation. The capacity of the rocks to retain cations is measured by the cation-exchange capacity (CEC), which is the sum of the quantity of positive charges (cations) neutralizing permanent and variable pH-dependent charges. The CEC strongly depends on the rock mineralogy, the surface area, and charge and size of the ion.36−39 Under steady-state chemical conditions, the composition of a cation exchanger is in equilibrium B
DOI: 10.1021/acsearthspacechem.6b00015 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
Article
ACS Earth and Space Chemistry as follows. In Section 2, we briefly present the reactions considered in this study. Next, we describe the experimental material, setup, and procedure in Section 3. In Section 4, the results of the experiments and their interpretations are discussed. We end the paper with the main conclusions of this study.
Table 1. Compositions of High-Salinity and Low-Salinity Brines (in mmol/L and mequiv/L) high-salinity brine (HSB)
2. CALCULATION OF CATION EXCHANGE CAPACITY In the simulations, we consider two main geochemical reactions, namely, cation exchange and dissolution of dolomite and calcite. The simulations were performed using Shell’s in-house transport simulator, MoReS,52 which is coupled to the chemistry package PHREEQC.47,53 PHREEQC is a software program with an extensive and editable database (with chemical reactions and their equilibrium constants) and can simulate chemical reactions to provide the composition of the aqueous phase in contact with minerals, gases, exchangers, and sorption surfaces.35 The default phreeqc.dat database file was included in our calculations, which uses Davies or Debye−Hückel equations for charged species. To model the ion exchange the amount of exchanger, denoted by X (= Qv , mequiv/mL of pore volume) and defined as master exchange species in the PHREEQC database, should be provided. The ion exchange reactions occur in two steps in PHREEQC, which uses mass-action expressions based on halfreactions between the aqueous species and a fictive unoccupied exchange site41 for each exchanger. The following exchange reactions were modeled in this study Na + + X = NaX
KNa =
Ca 2 + + 2X = CaX 2
[NaX] [Na +][X]
K Ca =
Mg 2 + + 2X = MgX 2
(3)
[MgX 2] [Mg 2 +][X]2
(4)
The combination of eq 2 with eq 3 and eq 4 provides the reactions that are more commonly used for modeling the exchange of sodium, calcium and magnesium ions 2NaX + Ca 2 + = CaX 2 + 2Na +
K Ca/Na =
[CaX 2][Na +]2 [Ca 2 +][NaX]2 (5)
2NaX + Mg 2 + = MgX 2 + 2Na +
KMg/Na =
[MgX 2][Na +]2 [Mg 2 +][NaX]2 (6)
KCa/K2Na
217 1.8 1.35 223.2
mequiv/L
217 3.6 2.7 223.2 223.3 0.03 3.15 6.3 275 mmol/L 20.4 nm
mmol/L
mequiv/L
35.5 2.55 0.775 42.15
35.5 5.1 1.55 42.15 42.15 0.16 3.325 6.65 136 mmol/L 26.4 nm
3. EXPERIMENTS 3.1. Chemicals. The brine used in the experiments was prepared by dissolving NaCl, MgCl2·6H2O, and CaCl2·2H2O (Fisher Scientific) in deionized water. The compositions of the high-salinity and low-salinity brines are shown in Table 1. The properties of the crude oils are listed in Table 2. 3.2. Core Samples. Berea sandstone cores were used to perform the experiments. The bulk mineral composition of Berea sandstone56,57 was characterized by XRD, the results of which are shown in Table 3. The geochemical reactions occur at the surface of the rock (and in particular clays because of their large surface area)36−39 and therefore bulk mineralogy is only a partial indicator of the presence of minerals that might influence the direction of the reactions. The cores had a diameter of 3.8 cm and length of 17.0 cm. The permeability and porosity of the cores were about 80− 120mD and 0.20 ± 0.1, respectively. The cores were cast in Araldite self-hardening glue to avoid production from the axial core sides. After hardening, the glue was machined so that the core fitted precisely in the core holder. One hole was drilled through the glue layer to the core surface to allow pressure measurements. The core-holder is made of poly(ethylene ether ketone) (PEEK).58−60 In our discussions, we will interchangeably use the terms “external” and “edge” surfaces whose charges are pH dependent. These charges occur on the clay edges because of the protonation/deprotonation of the surface hydroxyl groups and therefore may vary with the pH.61 Moreover, we use the terms “internal surface”, “basal surface”, and “faces” to refer to the sites with the permanent charges on the clay surfaces. These charges occur because of the isomorphous substitution of the cations with a larger valence by the cations with a smaller valence resulting in a net negative charge. Table 4 provides a summary of the main attributes of the clay surfaces. 3.3. Experimental Setup. The experimental setup is shown schematically in Figure 2. A P500 pump was used for injection of brine. A transfer vessel connected to another pump was used to inject crude oil into the core. The pressure transducers monitored the pressure-drop over the inlet and outlet. A backpressure regulator was mounted to set the outlet
[CaX 2]
KMg =
mmol/L
For larger pH variations (±0.5) hydrogen exchange should be considered. The chloride ion (Cl−) was used as the buffer to satisfy the charge balance errors and compensate for the anions that were not considered in our calculations. Moreover, dissolution of calcite and dolomite was considered in the calculations.
(2)
[Ca 2 +][X]2
component/description Na+ Ca2+ Mg2+ Cl− solution normality [N] fraction of divalents, [f2+] total divalent concentrations ionic strength Debye screening length
low-salinity brine (LSB)
KMg/K2Na.
where KCa/Na = and KMg/Na = In the simulations, we used log10 KNa = 0.0, log10 KCa = 0.77,54 and log10 Kmg = 0.60.55 Another important equation is the chargebalance equation, which sets the sum of the equivalent anions and cations, including those on the rock surface, to zero. We use the measured concentrations of the ions in the first effluent as the initial concentrations of the respective ions in the simulations. The measured concentrations of the injected lowsalinity brine were used as boundary condition, which were slightly different from the reported concentrations in Table 1 due to small variations in preparing the various solutions. We neglected the role of proton exchange in competition for exchange sites, which is justified because the pH variations were not large, as can be observed in the experimental results. C
DOI: 10.1021/acsearthspacechem.6b00015 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
Article
ACS Earth and Space Chemistry Table 2. Properties of the Crude Oils Used in the Experimentsa oil property
IFT with FW
API
density
viscosity
density
viscosity
TBN
TAN
unit crude oil A crude oil B
[mN/m] 30 27
[°API] 34.2 41.6
[g/cm3] (20 °C) 0.8540 0.8176
[mPa.s] (20 °C) 4.7765 3.2769
[g/cm3] (60 °C) 0.8153 0.7911
mPa·s (60 °C) 3.8216 1.6755
[mg KOH/g oil] 0.90
